A breath monitor for monitoring breath of a human while sleeping is disclosed in Japanese Patent Application Publication No. 2001-37742, which corresponds to U.S. Pat. No. 6,450,957. Specifically, the monitor is suitably used for diagnosing a patient having a respiratory disease. A sensor of the monitor is disposed under bedclothes to have a predetermined arrangement. The monitor includes multiple loading sensors for detecting an applied load of the patient. Each sensor outputs a load signal (i.e., a weight signal) as a sensor signal in accordance with the applied load. A breath signal is calculated from the weight signals such that a part of the weight signals having a certain frequency corresponding to a breathing rate of the patient is selected.
The monitor calculates the breath signal in detail as follows. Firstly, the weight signals outputted from the loading sensor are processed by a frequency analysis so that a power spectrum having a frequency range corresponding to the breathing rate is calculated. Then, one of the loading sensors, which has the maximum strength (i.e., maximum intensity) of the power spectrum, is selected as the first reference sensor. Another one of the sensors, which outputs the weight signal having the same phase or the reverse phase of the first reference sensor, is selected as the second reference sensor. When the second reference sensor outputs the weight signal having the same phase of the first reference sensor, the weight signals of the first and second reference sensors are added. When the second reference sensor outputs the weight signal having the reverse phase of the first reference sensor, the weight signal of the second reference sensor is reversed so that the weight signals of the first and second reference sensors are added. Thus, the breath signal is obtained by adding the weight signals.
However, the loading sensor may detect other loads caused by, for example, a motion of the patient instead of the load caused by the breathing. When the motion of the patient is larger than the breathing, the load by the motion is larger than that by the breathing. When the load by the motion is applied to one of the sensors, the one of the sensors may be selected as the first reference sensor. In this case, the monitor cannot detect the breath signal accurately.
Further, the breath monitor is suitably used for detecting apnea symptom of the patient so that a person such as a doctor or a nurse as an operator of the monitor diagnoses apnea syndrome of the patient while sleeping. It is required for the breath monitor to detect apnea symptom and/or hypopnea of the patient. However, the monitor according to the prior art cannot detect the hypopnea of the patient with high accuracy.